Johnson Creek Landslide, Coastal Oregon

The Johnson Creek landslide is located along the Oregon coast near the city of Newport. The landslide has a long history of impacting U.S. Highway 101, which passes over the middle section of the slide. The slide is up to 26 m thick, 200 m long, and 360 m wide, and occurs within the Astoria Formation of Miocene age. At the landslide location, the Astoria formation consists of siltstone, sandstone, mudstone and tuffaceous claystone, and dips 15-20 degrees to the west. Up to a 20 m wide graben defines the headscarp of the slide, and the toe daylights near the beach below the coastal cliff. Total movement of the slide, as estimated from geologic cross-sections, is 28 m horizontally and 6 m vertically. The most recent significant movement of the slide occurred in early 2002, when it moved approximately 25 cm horizontally and several centimeters vertically.

The Oregon Department of Geology and Mineral Industries (DOGAMI), supported by the Oregon Department of Transportation (ODOT), began researching the Johnson Creek landslide in November, 2002. Instrumentation installed at the site as part of the initial investigations consists of a tipping bucket raingage, vibrating-wire piezometers emplaced near the basal slip zone at three locations in the slide, and ¼-inch-diameter wire rope extensometer cables emplaced in three abandoned inclinometer borings each located near the piezometer installations. The extensometer cables at each boring are anchored in cement below the basal slip zone and extend up the inside of the inclinometer casings to a few meters beyond the ground surface. When the slide moves, the mass above the basal slip zone overrides the cable and pulls it into the borehole an amount approximately equal to the amount of landslide movement. Initially, the amount of movement was determined by periodically measuring the length of cable extending beyond the collar of the borings. Rainfall and pore pressures were recorded by small dataloggers at each borehole location. With the periodic manual measurement of the extensometer cables, however, it was not possible to precisely correlate the timing of slide movement with the pore-pressure values and rainfall.

In late 2004, the U.S. Geological Survey (USGS) Landslide Hazards Program began a cooperative effort with DOGAMI and ODOT to implement and conduct near real-time monitoring of the landslide in order to better characterize the relationships between precipitation, pore pressure and landslide movement over several seasonal cycles. Automatic data acquisition systems were installed to monitor the existing instruments, and cable-extension transducers were attached to the extensometer cables such that landslide movement could be measured simultaneously with precipitation and groundwater pressure. Subsequently, cell phone telemetry was added to the systems such that data could be collected remotely without the necessity of site visits. Rainfall, groundwater pressures, and landslide movement are now monitored on 15-minute intervals and transmitted daily by the cell-phone link to USGS offices in Golden, Colorado.

The most recent data for the past four weeks are displayed on graphs updated daily and posted to this web site and include

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